Hologram recording method using beam with very large incident angle, hologram reproduction apparatus using holographic reflector and sologram reproduction method using the same, and flat display element apparatus using holographic reflector

ABSTRACT

A hologram recording method using a beam with a very large incident angle includes the steps of generating a sheet beam as the reference beam, and introducing the reference beam into the recording medium at an incident angle of at least 70°. A hologram reproduction apparatus using a holographic reflector includes a light, a holographic reflector, an adjustor, and a hologram. Also, the hologram reproduction method using the hologram reproduction apparatus includes the steps of irradiating a beam emitted from a light source as a reference beam to a holographic reflector, reflecting the reference beam illuminated to the holographic reflector therefrom to generate a reconstructed beam, and allowing the generated reconstructed beam as a reference beam to pass through a hologram to reproduce the hologram. A flat display element apparatus comprises a light source, a holographic reflector, an adjustor, and a flat display element.

Priority is claimed to Patent Application Number 2001-24381 filed inRep. of Korea on May 4, 2001, herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hologram recoding method, a hologramreproduction apparatus, a hologram reproduction method using the sameand a flat display element apparatus, and more particularly, to ahologram recording method using a beam with a very large incident angle,a hologram reproduction apparatus using a holographic reflector, ahologram reproduction method using the same and a flat display elementapparatus using a holographic reflector.

2. Description of the Related Art

A hologram refers to a three dimensional (3D) photographic imagephotograph created by means of a holography through a laser beam, whichis produced in a such a fashion that a reference beam or one beam of twolaser beams into which is divided by a beam splitter, and an object beamor the other divided by the beam splitter and then directed to anddiffused from an object of interest to be recorded as information areprojected onto a recording medium to record, in a photographic form, alarge number of minute inference fringes produced by the interaction ofthe reference beam and the object beam on the surface of the recordingmedium so as to form an image of the original object.

Dennis Garbor has originally invented a hologram in 1948. From that timeup to now, most widely used types of holograms include a two-beamtransmission hologram developed by E. Leith et al., an one-beamreflection hologram developed by Denisyuk et al., and an edge-lithologram developed by N. Phillips et al., and the like.

For such a transmission hologram, in the recording of it, all thereference and object beams are projected onto a recording medium in thesame direction with respect to the recording medium to produce ahologram, and in the reproduction of it, a reference beam with samewavelength as that of the reference beam projected in the recording ofthe hologram illuminates the surface of the recording medium on whichthe hologram has been recorded, at the same incident angle as that inthe recording of the hologram, while passing through the recordingmedium to obtain a reproduction beam to reconstruct the hologram in adirection opposite to the incident direction of the reference beamthrough diffraction and diffusion of the reproduction beam due to theinterference fringes so as to form an image of the original object. Atthis point, the reproduction of the hologram means that informationrecorded on the hologram is represented visually.

For such a reflection hologram, in the recording of it, the referencebeam and the object beam are projected onto the recording medium in thedirection opposite to each other, in the reproduction of it, a referencebeam with same wavelength as that of the reference beam in the recordingof the hologram illuminates the surface of the recording medium on whichthe hologram has been recorded, at the same incident angle as that inthe recording of the hologram, while being reflected by the recordingmedium to obtain a reproduction beam to reconstruct the hologram in thesame direction as the incident direction of the reference beam so as toform an image of the original object.

FIG. 1 is a schematic view illustrating a general transmission typehologram recording method according to the prior art.

As shown in FIG. 1, a reference beam 11 and an object beam 13 aresimultaneously projected onto a recording medium 15 in such a fashionthat they are identical with each other in their incident directions.

FIG. 2 is a schematic view illustrating a general reflection typehologram recording method according to the prior art.

As shown in FIG. 2, a reference beam 11 and an object beam 13 aresimultaneously projected onto a recording medium 15 in such a fashionthat they are opposite to each other in their incident directions. Inorder to obtain a maximum efficiency in the recording of the hologram,the incident angles of both the reference and the object beams must bemaintained at a predetermined angle. For this reason, an incident angleof a reference beam in the reproduction of the hologram must also bemaintained at the predetermined angle to be identical with that of thereference beam in the recording of the hologram.

FIG. 3 is a schematic view illustrating a general transmission typehologram reproduction method according to the prior art.

As shown in FIG. 3, a reference beam 11 with the same wavelength andincident angle as those of the reference beam in the recording of thehologram illuminates the surface of the recording medium on which thehologram 17 has been recorded while passing through the recording mediumto obtain a reproduction beam to reconstruct the hologram 17 in adirection opposite to the incident direction of the reference beamthrough diffraction and diffusion of the reproduction beam due to theinterference fringes so as to form an image of the original object.

FIG. 4 is a schematic view illustrating a general reflection typehologram reproduction method according to the prior art.

As shown in FIG. 4, a reference beam 11 with the same wavelength andincident angle as those of the reference beam in the recording of thehologram illuminates the surface of the recording medium on which thehologram 17 has been recorded, at the same position as that in therecording of the hologram, while being reflected from the recordingmedium to obtain a reproduction beam to reconstruct the hologram 17 inthe same direction as the incident direction of the reference beam so asto form an image of the original object.

FIG. 5 is a schematic view illustrating a color separation due to adiffraction of a reference beam generated in the reproduction of atransmission type hologram according to the prior art. A reproductionbeam 19 formed by allowing the reference beam 11 to pass through orreflected by the recording medium on which the hologram 17 is recordedreproduces information recorded on the hologram 17, which makes itpossible for an observer to view it in a stereoscopic or planar form.

In case of reproduction of a hologram based on such conventionalhologram recording methods, a light source must be provided separatelyand the same condition as that in the recording of the hologram must beprepared. Accordingly, the conventional hologram reproduction methodsare accompanied by the restraints on a place where the reproduction ofthe hologram is implemented. Also, in case of a holographic opticalelement (hereinafter, referred to as “HOE”), the size of an entiresystem becomes larger. Particularly, since the condition of irradiationfor the reference beam at the time of reproducing a transmission typehologram must be identical with that of irradiation for the referencebeam at the time of recording it, there may occur a problem in the placewhere a hologram is reproduced or the structure of a hologramreproduction apparatus.

In particular, as shown in FIG. 5, in the case where the hologram 17 orHOE has been recorded on a recording medium, and then is reproduced in afull color image form, a reproduction beam 19 is separated depending onits wavelength according to an irradiation angle of the reference beam11.

In order to resolve this problem, in the recording of the hologram, amirror is often used. But, this approach has a disadvantage in that itis not easy to adjust an angle of reflection of the mirror. Also, incase of employing other gratings or HOE, there occurs a color separationaccording to wavelengths of light beams, and the characteristics of anoise are insufficient. In particular, in case of producing a hologramin a full color image form, efficiency is degraded.

In addition, for a conventional edge-lit hologram, it is verycomplicated to record a hologram and efficiency is degraded.

In general, it has been known that when a full color hologram or HOE isrecorded on one recording medium, efficiency is lowered to ½, andreliability of color reproduction is decreased.

Further, it is difficult to adjust a hologram reproduction angle(reflection angle) by color.

SUMMARY OF THE INVENTION

To solve the above-described problems, it is an object of the presentinvention to provide a hologram recording method in which no restraintsare imposed on the place where a hologram is reproduced or the structureof a hologram reproduction apparatus and it is simpler to reproduce thehologram.

It is another object of the present invention to provide a hologramreproduction apparatus and hologram reproduction method using thehologram reproduction apparatus with a more compact structure, in which,in the reproduction of a hologram, a recording medium can be illuminatedat the same irradiation angle as that of a reference beam in therecording of a hologram irrespective of the types of holograms to obtaina reproduction beam.

It is still another object of the present invention to provide ahologram reproduction apparatus with a simpler structure and hologramreproduction method using the hologram production apparatus, in which anillumination device is configured using a holographic reflector so thateven in case of a polychromatic light source, a reconstruction angle ofa reconstructed beam is maintained equally, thereby resulting in aprevention of a color separation, and which can achieve improvementsboth in transmittance of wavelengths of beams and in reproductionefficiency of a hologram.

It is yet another object of the present invention to provide a flatdisplay element apparatus which is provided with an irradiation devicehaving a smaller variation in a color temperature according towavelengths of beams, thereby improving transmittance of wavelengths ofthe beams and reproduction efficiency of a hologram.

In order to accomplish this object, there is provided a method ofrecording a hologram in which a reference beam and an object beam areintroduced into a recording medium, comprising the steps of generating asheet beam as the reference beam, introducing the reference beam intothe recording medium at an angle of at least 70°. In this case, a singlebeam may be used as the reference beam and the object beam.

The generating step may further include the steps of opticallyprocessing a point beam emitted from a certain light source to generatea linear beam, and optically processing the linear beam to generate thesheet beam. Here, the step of optically processing a point beam togenerate a linear beam is a step of shaping a point beam using anappropriate optical means to generate a linear beam.

It is preferred that the step of optically processing the point beam mayfurther include the step of allowing the point beam to pass through acylindrical lens or to be reflected by a cylindrical surface mirror togenerate the linear beam. Preferably, the step of optically processingthe linear beam may further include the step of allowing the linear beamto pass through a semi-cylindrical lens so that a line width of thelinear beam is adjusted to generate the sheet beam.

Further, the reference beam as the sheet beam may be introduced directlyinto the recording medium or may be introduced indirectly into therecording medium through the reflection of a mirror. The object beam maybe introduced directly into the recording medium or may pass through asemitransparent mirror, and then may be introduced into the recordingmedium.

According to another aspect of the present invention, there is alsoprovided an apparatus for reproducing a hologram, comprising a lightsource for emitting light, a holographic reflector for receiving, as areference beam, a beam emitted from the light source and reflecting areference beam therefrom to generate a reconstructed beam, an adjustorfor adjusting an irradiation angle of the reference beam directed to theholographic reflector from the light source, and a hologram forreceiving, as the reference beam, the reconstructed beam reflected bythe holographic reflector.

It is preferred that the hologram may include a transmission typehologram recorded in such a fashion that the reference beam isintroduced into the hologram at an incident angle of at least 70°.Preferably, the holographic reflector may include at least one selectedfrom the group of three holographic reflectors recorded with beams ofred, green and blue colors.

Further, the holographic reflector is constructed in such a fashion thatthe holographic reflector recorded with a beam of the blue color isarranged in the front of the three holographic reflectors so that it isfirst illuminated when viewed in the irradiation direction of thereference beam.

Also, the light source may include any one selected from the group of alinear metal lamp, a fluorescent lamp, a cold cathode fluorescent tube(hereinafter, referred to as “CCFT”) and an LED array.

It is preferred that the adjustor may include a concave reflecting platedisposed at the rear side of the light source, or a cylindrical lens ora light diffusing plate disposed at the front side of the light source.A wedge prism or an optical flat element may further be disposed betweenthe holographic reflector and the hologram.

According to another aspect of the present invention, there is alsoprovided a method of reproducing a hologram, comprising the steps of:

-   -   irradiating a beam emitted from a light source as a reference        beam to a holographic reflector;    -   reflecting the reference beam illuminated to the holographic        reflector therefrom to generate a reconstructed beam; and    -   allowing the generated reconstructed beam as a reference beam to        pass through a hologram to reproduce the hologram.

It is preferred that the irradiating step may further include the stepof allowing the reference beam illuminated to the holographic reflectorfrom the light source to be reflected by a concave reflecting platedisposed at the rear side of the light source.

It is preferred that the beam irradiating step may further include thestep of allowing the reference beam illuminated to the holographicreflector from the light source to pass through a cylindrical lens or alight diffusing plate disposed at the front side of the light source.

It is also preferred that in the reference beam reflecting step, anemitting angle of the reconstructed beam from the holographic reflectormay be adjusted according to a wavelength of the reflected referencebeam.

Preferably, in the allowing step, a wedge prism or an optical flatelement may further be disposed between the holographic reflector andthe hologram.

Also, the hologram may include a transmission type hologram recorded insuch a fashion that the reference beam is introduced into the hologramat an incident angle of at least 70°.

According to another aspect of the present invention, there is alsoprovided a flat display element apparatus, comprising:

-   -   a light source for emitting light;    -   a holographic reflector for receiving, as a reference beam, a        beam emitted from the light source and reflecting the reference        beam therefrom to generate a reconstructed beam;    -   an adjustor for adjusting an irradiation angle of the reference        beam directed to the holographic reflector from the light        source; and    -   a flat display element for receiving, as a reference beam, the        reconstructed beam reflected by the holographic reflector.

Preferably, the holographic reflector may include at least one selectedfrom the group of three holographic reflectors recorded with beams ofred, green and blue colors.

Further, the holographic reflector is constructed in such a fashion thatthe holographic reflector recorded with a beam of the blue color isarranged in the front of the three holographic reflectors so that it isfirst illuminated when viewed in the irradiation direction of thereference beam.

Also, the light source may include any one selected from the group of alinear metal lamp, a fluorescent lamp, a cold cathode fluorescent tube(hereinafter, referred to as “CCFT”) and an LED array.

It is preferred that the adjustor may include a concave reflecting platedisposed at the rear side of the light source, and a cylindrical lens ora light diffusing plate disposed at the front side of the light source.A wedge prism or an optical flat element may further be disposed betweenthe holographic reflector and the hologram.

It is preferred that the flat display element may include an LCD. Inthis case, the structures and functions of the holographic reflector,the light source and the adjustor have the same those as described in ahologram reproduction apparatus using a holographic reflector and ahologram reproduction method using the same.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore apparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a schematic view illustrating a general transmission typehologram recording method according to the prior art;

FIG. 2 is a schematic view illustrating a general reflection typehologram recording method according to the prior art;

FIG. 3 is a schematic view illustrating a general transmission typehologram reproduction method according to the prior art;

FIG. 4 is a schematic view illustrating a general reflection typehologram reproduction method according to the prior art;

FIG. 5 is a schematic view illustrating a color separation due to adiffraction of a reference beam generated in the reproduction of atransmission type hologram according to the prior art;

FIG. 6 is a schematic view illustrating a transmission type hologramrecording method according to a first embodiment of the presentinvention;

FIG. 7 is a schematic view illustrating a reflection type hologramrecording method according to a first embodiment of the presentinvention;

FIG. 8 is a schematic view illustrating a transmission type hologramrecording method according to a second embodiment of the presentinvention;

FIG. 9 is a schematic view illustrating a reflection type hologramrecording method according to a second embodiment of the presentinvention;

FIG. 10 is a schematic perspective view illustrating a method ofproducing a sheet beam using a cylindrical lens in a hologram recordingmethod according to an embodiment of the present invention;

FIG. 11 is a schematic perspective view illustrating a method ofproducing a sheet beam using a cylindrical surface mirror in a hologramrecording method according to an embodiment of the present invention;

FIG. 12 is a sectional side view illustrating a hologram reproductionapparatus according to a first embodiment of the present invention;

FIG. 13 is a side view illustrating a transmission type hologramreproduction apparatus according to a second embodiment of the presentinvention;

FIG. 14 is a schematic side view illustrating a transmission typehologram reproduction apparatus according to a second embodiment of thepresent invention;

FIG. 15 is a side view illustrating a flat display element apparatusconstituting an LCD back light according to a first embodiment of thepresent invention;

FIG. 16 is a schematic side view illustrating the flat display elementapparatus of FIG. 15;

FIG. 17 is a schematic view illustrating a transmission type hologramreproduction method according to a first embodiment of the presentinvention;

FIG. 18 is a schematic view illustrating a reflection type hologramreproduction method according to a first embodiment of the presentinvention;

FIG. 19 is a graph illustrating a variation in transmittance of ahologram depending on different wavelengths of beams in a hologramrecording method according to the present invention; and

FIG. 20 is a table illustrating a variation in reflectance andtransmittance of a hologram depending on different wavelengths of beams.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a hologram recording method, a hologram reproductionapparatus and a hologram reproduction method using the same, and flatdisplay element apparatus according to preferred embodiments of thepresent invention will be in detail described with reference to theaccompanying drawings.

First, a hologram recording method according to preferred embodiments ofthe present invention will be described hereinafter.

FIG. 6 is a schematic view illustrating a transmission type hologramrecording method according to a first embodiment of the presentinvention.

Referring to FIG. 6, a reference beam 51 is projected directly onto arecording medium 55 at a large angle of more than 70° or is projectedindirectly onto the recording medium 55 through the reflection of amirror 56 and, simultaneously, an object beam 53 is projected directlyonto the recording medium 55 or projected indirectly onto the recordingmedium 55 through reflection of a semitransparent mirror 58 in the sameplane as that of the recording medium 55 onto which the reference beam51 is projected. In this point, the object beam 53 refers to a lightbeam that is scattered from an object 57 to be recorded on the recordingmedium 55. The recording medium 55 becomes a transmission hologram orHOE for recording information about the object.

FIG. 7 is a schematic view illustrating a reflection type hologramrecording method according to a first embodiment of the presentinvention.

Referring to FIG. 7, the reflection type hologram recording method isdifferent from the transmission type hologram recording method in thatthe reference beam 51 and the object beam 53 are projected onto therecording medium in the direction opposite to each other. However, thereflection type hologram recording method is identical with thetransmission type hologram recording method in a manner in which thereference beam 51 and the object beam 53 are projected onto therecording medium 55.

In the above hologram recording methods according to the presentinvention, the reference beam 51 is projected onto the recording medium55 at a large angle of more than 70° to produce a hologram, so that amore compact hologram reproduction apparatus can be implemented. Forexample, for a hologram or a holographic optical element (HOE) recordingmethod in which a reference beam has a large incident angle of more than70°, if the hologram or HOE is recorded on one recording medium in onewavelength form, a high efficiency of more than 90% can be obtained atthe time of reproduction of the hologram, and the wavelength and angleof the beam reproduced can also be easily adjusted.

FIG. 8 is a schematic view illustrating a transmission type hologramrecording method of the hologram recording methods using a single beamwith a large incident angle, according to a second embodiment of thepresent invention.

Referring to FIG. 8, a reference beam 51 is reflected by asemitransparent mirror 58 to be directed into a recording medium 55,while passing through the semitransparent mirror 58 to be directed to aconcave mirror 54 by which the reference beam, in turn, is reflected sothat an object beam 53 is generated. The reference beam 51 and theobject beam 53 are directed to the recording medium 55 in such a fashionthat they are identical with each other in their incident directions. Inthis case, the reference beam 51 as a sheet beam is introduced into therecording medium 55 at a large angle of more than 70°.

FIG. 9 is a schematic view illustrating a reflection type hologramrecording method of the hologram recording methods using a single beamwith a large incident angle, according to a second embodiment of thepresent invention. Referring to FIG. 9, a reference beam 51 is directedinto a recording medium 15, while passing through the recording medium55 to be directed to a concave mirror 54 by which the reference beam, inturn, is reflected so that an object beam 53 is generated. The referencebeam 51 and the object beam 53 are directed to the recording medium 55in such a fashion that they are opposite to each other in their incidentdirections. In this case, the reference beam 51 as a sheet beam 31 isintroduced into the recording medium 55 at a large angle of more than70°. The incident angle of the reference beam 51 is preferably set at alarge angle of more than 70° in order to make an optical system compact.

Generally, in case of employing one beam instead of using more than twobeams, the recording of a hologram can be done more simply. However, inthe prior art, there have been problems in that, for the sake ofconvenience of irradiation in the reproduction of the hologram, areference beam must be introduced into a recording medium at theincident angle of 45°. Also, since the reproduction of the hologramrequires the use of the same irradiation angle as the incident angle,i.e., 45° of the reference beam in the recording of the hologram, thedimensions of an entire hologram reproduction system becomes larger. Forthe reflection type hologram recorded and reproduced using a single beamaccording to the present invention, since it is recorded through the useof a reference beam having a large incident angle of more than 70, ahologram reproduction apparatus of a simpler structure can beimplemented.

FIG. 10 is a schematic perspective view illustrating a method ofproducing a sheet beam using a cylindrical lens in a hologram recordingmethod according to an embodiment of the present invention.

Referring to FIG. 10, there is shown a process for generating a sheetbeam 31 as the reference beam 51. In this process, first, a point beam23 emitted from a point light source passes through a cylindrical lens25 to generate a linear beam 27 which, in turn, passes through asemi-cylindrical lens 29 so that a line width of the linear beam 27 isadjusted to generate the sheet beam 31.

FIG. 11 is a schematic perspective view illustrating a method ofproducing a sheet beam using a cylindrical surface mirror in a hologramrecording method according to an embodiment of the present invention.

Referring to FIG. 11, there is shown a process for generating a sheetbeam 31 as the reference beam 51. In this process, first, a point beam23 emitted from a point light source is directed to a cylindricalsurface mirror 33 by which the point beam 23 is reflected to generate alinear beam 27 which, in turn, passes through a semi-cylindrical lens 29so that a line width of the linear beam 27 is adjusted to generate thesheet beam 31 which is projected uniformly onto the recording medium 15.

The aim of using the sheet beam 31 as the reference beam 51 is toimprove reproduction efficiency in the reproduction of a hologram byallowing the reference beam to uniformly illuminate the surface of therecording medium 55. Since respective portions on the hologram possessall information about an object of interest to be recorded on therecording medium, it is important that the information such as the phaseand amplitude of an object beam scattered from the object should berecorded uniformly and distinctly on the recording medium. The moreuniformly the information is recorded on the recording medium, the moredistinctly an image of the object appears at the time of reproduction ofthe hologram and the greater reproduction efficiency of the hologram isimproved.

Now, a hologram reproduction apparatus according to the presentinvention will be described in detail hereinafter.

FIG. 12 is a sectional side view illustrating a hologram reproductionapparatus according to a first embodiment of the present invention.

As shown in FIG. 12, the hologram reproduction apparatus according tothe present invention includes a light source 35, a recording medium 67for receiving, as a reference beam, a light beam emitted from the lightsource 35, a hologram being recorded on the recording medium 67, and asupport 37 for supporting both sides of the recording medium 67, thesupport having the light source 35 installed at one side thereof.

There is disposed a cylindrical lens or a light diffusing plate at thefront side of the light source 35, or a concave reflecting plate isdisposed at the rear side of the light source 35 for adjusting anillumination angle and an illumination area of a reference beamintroduced into the recording medium 67. In order to obtain a clearreproduction beam, the illumination angle of the reference beam in thereproduction of the hologram must be adjusted depending on the incidentangle of a reference beam in the recording of the hologram.

The hologram recorded on the recording medium 67 may be a transmissiontype hologram or a reflection type hologram. In case of the transmissiontype hologram, a beam as a reference beam emitted from the light source35 passes through the recording medium 67 to generate a reconstructedbeam which is diffused in a direction opposite to the incident directionof the reference beam, i.e., in the upward direction of the drawingsheet to reproduce the hologram so as to form an image of an originalobject. On the other hand, in case of the reflection type hologram, thereference beam is reflected by the recording medium 67 to generate areconstructed beam which is diffused in the same direction as theincident direction of the reference beam, i.e., in the downwarddirection of the drawing sheet to reconstruct the hologram.

The support 37 includes the light source 35 installed at one sidethereof, and acts to support both sides of the recording medium 67.Also, the hologram reproduction apparatus may include a holographicreflector 39 arranged on the optical path of the beam directed to therecording medium 67 from the light source 35.

The holographic reflector 39 enables the irradiation angle of thereference beam to be easily adjusted in the reproduction of thetransmission type hologram. In case of the reproduction of apolychromatic hologram, a hologram reproduction apparatus of a simplerstructure can be manufactured and used for preventing a dispersion ofcolors.

FIG. 13 is a side view illustrating a transmission type hologramreproduction apparatus employing a holographic reflector according to asecond embodiment of the present invention.

As shown in FIG. 13, the transmission type hologram reproductionapparatus using a holographic reflector includes a light source 35, aholographic reflector 39 for receiving, as a reference beam, a beamemitted from the light source 35 and reflecting the reference beamtherefrom to generate a reconstructed beam, and a recording medium 41for receiving, as a reference beam, the reconstructed beam reflected bythe holographic reflector 39 and allowing the received reference beam topass therethrough to generate a reproduction beam so as to reproduce atransmission type hologram.

The reference beam 61 emitted from the light source 35 is reflected bythe holographic reflector 39 which allows the reflected beam toregenerate and diverge at different reconstruction angles according toeach of the wavelengths of light beams with which the holographicreflector 61 is recorded to direct to the recording medium 41. The beamsreflected by the holographic reflector 39 are adjusted such that theirreconstruction angles become equal to each other, which illuminate thesurface of the recording medium 41 so that a reproduction beam isobtained to reproduce the transmission type hologram in a directionopposite to the incident direction of the reference beam throughdiffraction and diffusion of the reproduction beam due to theinterference fringes so as to form an image of the original object.

The aim of making the different reconstruction angles of the beamreflected and regenerated by the holographic reflector 39 identical witheach other is to minimize a dispersion effect of colors due toreflective indexes and diffractions according to different wavelengthsof the reproduction beam at the time of reproduction of the transmissiontype hologram or HOE.

A principle of adjusting the reconstruction angle of the reference beamreflected and diverged by the holographic reflector 39 will now bedescribed in detail hereinafter.

When the reference beam reflected by the holographic reflector 39illuminates on the surface of the recording medium 41 on which thetransmission type hologram or HOE is recorded, beams of red, green andblue colors are reproduced at a predetermined angle in any particulardirection so that a reproduction beam is obtained to reproduce thehologram.

When a transmission type holographic reflector is recorded with a greenbeam the wavelength and incident angle of which is λG and θG,respectively, the spacing dG between adjacent gratings of thetransmission type holographic reflector can be written as the following[Expression 1].dG=λG/(2 sin θG)  [Expression 1]

As shown in the above [Expression 1], in the case where the gratings inthe holographic reflector as recorded above are regenerated to become ared color and a blue color, a red color reconstruction angle and a bluecolor reconstruction angle of a beam to be regenerated are given by thefollowing [Expression 2] and [Expression 3], respectively.θR=sin−1(λR/2dG)  [Expression 2]θB=sin−1(λB/2dG)  [Expression 3]

-   -   where θR is the red color reconstruction angle, and θB is the        blue color reconstruction angle.

When the reflection angles of holographic reflectors at the back side inthe holographic reflector 39 of a laminated structure are adjustedsuitably according to the above reconstruction angles so that all thereconstruction angles of the three color wavelengths become θG, aregeneration of a white light is possible. When the holographicreflector 39 for regenerating three colors is fixed to a base plate andthe holographic reflector 39 is illuminated by a reference beam 11 usinga light source, a reflective plate lens and the like, a white light isregenerated in a desired direction as shown in FIG. 17.

Using such a reconstruction angle adjusting principle, a compacttransmission type hologram reproduction apparatus can be manufactured asshown in FIG. 14.

In case of a general light source, a beam of a blue color has a lowerbrightness or intensity. For this reason, although all the beams of red,green and blue colors in a spectrum of a light source are identical withone another in a light intensity, a beam of a shorter wavelength is morerapidly absorbed or degraded in transmittance in terms of thecharacteristics of a base plate used in an optical instrument, ahologram or a display, etc. In order to address this problem, whenmanufacturing the holographic reflector 39 recorded with beams of threecolors of red, green and blue, the holographic reflector 39 of astructure laminated in the order of blue, red and green colors is usedwhen viewed in the irradiation direction of the reference beam 61 to beregenerated. Even in case of using a laminated holographic reflectorrecorded with beams of two colors including a blue color among the threecolors, the holographic reflector recorded with a beam of the blue coloris arranged in the front of the laminated holographic reflector whenviewed in the irradiation direction of the beam to be regenerated sothat the blue color recorded holographic reflector is first illuminated,thereby improving a reproduction efficiency of a blue beam.

FIG. 14 is a schematic side view illustrating a transmission typehologram reproduction apparatus using a holographic reflector shown inFIG. 13.

In the above embodiment, a light source 35 may include a linear metallamp, a fluorescent lamp, a CCFT or an LED array. An optical system asan adjustor for adjusting the shape of a beam, i.e., an irradiation areaand irradiation angle of a beam to illuminate the transmission typehologram 41 is identical with the above described transmission typehologram reproduction apparatus.

Next, a flat display element apparatus according to a first embodimentof the present invention will be described hereinafter with reference toFIG. 15.

FIG. 15 is a side view illustrating a flat display element apparatusconstituting an LCD back light according to a first embodiment of thepresent invention.

Referring to FIG. 15, an LCD back light according to the presentinvention includes a light source 35, a holographic reflector 39 and anLCD 43. The structures of the light source 35 and the holographicreflector 39 are the same as those of the light source 35 and theholographic reflector 39 as shown in FIGS. 13 and 14 except that thehologram 41 as the recording medium in FIGS. 13 and 14 is replaced withthe LCD 43 in FIG. 15, which have already been described. A wedge prism45 may further be disposed between the holographic reflector 39 and theLCD 43. The wedge prism 45 serves to maintain the function of anirradiation section and define an optical path. Also, a function fordiffusing a beam or a light diffusing plate can be added on the surfaceof the wedge prism 45 to obtain a more uniform irradiation.

The holographic reflector 39 is fixed to one end of the wedge prism 45at an inclined angle and the light source 35 is disposed at one edge ofthe wedge prism 45. The beam emitted from the light source 35 isdirected to the holographic reflector 39 using a concave mirror or alens. An illumination beam reflected by the holographic reflector 39passes through the wedge prism 45 while being diverged to illuminate theLCD 43 so that an LCD back light is implemented.

FIG. 16 is a schematic side view illustrating the LCD back light of FIG.15.

Referring to FIG. 16, it can be seen that an adjustment of thereconstruction angle of the holographic reflector 39 makes thereconstruction angles of beams according to different wavelengthsidentical. This illumination angle adjustment principle has already beendescribed above.

FIG. 17 is a schematic view illustrating a transmission type hologramreproduction method according to a first embodiment of the presentinvention.

FIG. 18 is a schematic view illustrating a reflection type hologramreproduction method according to a first embodiment of the presentinvention.

Referring to FIG. 17, the above hologram reproduction method isidentical with a convention hologram reproduction method except that incase of a hologram recorded according to the present invention, since areference beam is directed to the hologram 87 at an incident angle ofmore than 70° at the time of the recording of the hologram 87, at thetime of reproduction of the hologram, the reference beam must also bedirected to the holographic reflector 39 at the same incident angle asthat at the time of the recording of the hologram.

In case of the transmission type hologram, the reference beam 81reflected by the holographic reflector 39 illuminates the hologram 87 atthe same incident angle as that in the recording of the hologram 87while passing through it so that a reproduction beam 89 is generated toreproduce the hologram 87 in a direction opposite to the incidentdirection of the reference beam through diffraction and diffusion of thereproduction beam due to the interference fringes so as to form an imageof the original object. On the other hand, in case of the reflectiontype hologram, the reference beam 81 reflected by the holographicreflector 39 illuminates the hologram 87 at the same incident angle asthat in the recording of the hologram while being reflected by thehologram 87 so that a reproduction beam 89 is generated to reproduce thehologram 87 in the same direction as the incident direction of thereference beam so as to form an image of the original object.

Also, in case of a hologram reproduction method using a single beam, thesame hologram reproduction method as the above described hologramreproduction method is used.

FIG. 19 is a graph illustrating a variation in transmittance of ahologram depending on different wavelengths of beams in a hologramrecording method according to the present invention.

Referring to FIG. 19, as a result of the manufacture of HOE to which areference beam is directed at a larger incident angle, satisfactoryresults have been obtained in view of an illumination angle, wavelengthsof regeneration beams and a hologram reproduction efficiency.

Referring to FIG. 19, in a reflection type HOE in which PFG-03C is usedas a recording medium, respective values of transmittance andreflectance with respect to a red wavelength (647 nm), a greenwavelength (532) and a blue wavelength (458) are depicted. It can beseen from the graph of FIG. 19 that when blue, green and red wavelengthsare 457.22 nm, 531.5 nm and 647.81 nm, respectively, in a horizontal (X)axis, all the values of transmittance of beams with respect to the threecolor wavelengths are below 1%, while all the values of reflectance ofbeams of the three colors are high.

FIG. 20 is a table illustrating a variation in reflectance andtransmittance of a hologram depending on different wavelengths of beams.

Referring to FIG. 20, it can be seen from the table of FIG. 20 that incase of a silver halide sensitized gelatin (SHSG) reflection type HOE,when blue, green and red wavelengths are 458 nm, 532 nm and 647 nm,respectively, the values of reflectance of beams with respect to thethree color wavelengths are 96.3%, 96.5% and 96.8%, respectively, whilethe values of transmittance of beams of the three colors are 0.2%, 0.5%and 0.8%, respectively. Accordingly, in the case where a reference beamis directed to a holographic reflector at a large incident angle, ahologram with a very high reproduction efficiency can be manufactured.

As described above, a hologram recorded and reproduced according to thepresent invention has an advantage in that in case of reproduction ofthe hologram, a compacter holographic system or holographic display canbe manufactured without a limitation on a hologram reproduction place orthe structure of a hologram reproduction apparatus. Further, the use ofa holographic reflector prevents a color dispersion due to a diffractionof a reconstructed beam generated on a general transmission typehologram or HOE so that a white light reflected by the holographicreflector is used as an illumination beam to implement a reproduction ofa polychromatic beam. Furthermore, a hologram recording and reproductionmethod according to the present invention makes it possible tomanufacture a highly efficient LCD back light.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A method of recording a hologram in which a reference beam and anobject beam are introduced into a recording medium, comprising the stepsof: generating a sheet beam as the reference beam; and introducing thereference beam into the recording medium at an incident angle of atleast 70°, wherein a single beam is used as the reference beam and theobject beam.
 2. The method as claimed in claim 1, wherein the sheet beamgenerating step further comprises the steps of: optically processing apoint beam emitted from a certain light source to generate a linearbeam; and optically processing the linear beam to generate the sheetbeam.
 3. The method as claimed in claim 2, wherein the point beamprocessing step further comprises the step of allowing the point beam topass through a cylindrical lens to generate the linear beam.
 4. Themethod as claimed in claim 3, wherein the linear beam processing stepfurther comprises the step of allowing the linear beam to pass through asemi-cylindrical lens so that a line width of the linear beam isadjusted to generate the sheet beam.
 5. The method as claimed in claim2, wherein the point beam processing step further comprises the step ofallowing the point beam to be reflected by a cylindrical surface mirrorto generate the linear beam.
 6. The method as claimed in claim 5,wherein the linear beam processing step further comprises the step ofallowing the linear beam to pass through a semi-cylindrical lens so thata line width of the linear beam is adjusted to generate the sheet beam.7. The method as claimed in claim 1, wherein the object beam passesthrough a semitransparent mirror, and then is introduced into therecording medium.
 8. A method of recording a hologram in which areference beam and an object beam are introduced into a recordingmedium, comprising the steps of: generating a sheet beam as thereference beam, wherein the sheet beam is generated by passing a pointbeam through a cylindrical lens to generate a linear beam, and passingthe linear beam through a semi-cylindrical lens to generate the sheetbeam; and introducing the reference beam into the recording medium at anincident angle of at least 70°; wherein a single beam is used as thereference beam and the object beam.
 9. The method as claimed in claim 8,wherein the object beam passes through a semitransparent mirror, andthen is introduced into the recording medium.
 10. A method of recordinga hologram in which a reference beam and an object beam are introducedinto a recording medium, comprising the steps of: generating a sheetbeam as the reference beam, wherein the sheet beam is generated byreflecting a point beam by a cylindrical mirror to generate a linearbeam, and passing the linear beam through a semi-cylindrical lens togenerate the sheet beam; and introducing the reference beam into therecording medium at an incident angle of at least 70°; wherein a singlebeam is used as the reference beam and the object beam.
 11. The methodas claimed in claim 10, wherein the object beam passes through asemitransparent mirror, and then is introduced into the recordingmedium.